1. Field of the Invention
The present invention relates to a tubular structure formed from a straight strip bent or corrugated and the method of manufacturing same. It applies more particularly to the insulation of pipe lines serving more particularly for transporting hot or cold fluids, such as oil products (heavy oil or natural liquid gas), this structure being lagged with materials having heat insulation properties such for example as foams.
2. Description of the prior art
In the following description, the case of a pipe line will be considered by way of example in which the inner surface is formed by an internal sheath or tube.
It is known to form heat insulated pipe lines by causing a cellular product, such as a polyurethane foam, to expand between a mechanically tough tubular duct and another duct which will subsequently serve as protection.
The first duct will be called internal tube and the second external tube in the rest of the description.
A layer of glass wool, rock wool or another insulator may also be deposited on the surface of the internal tube, which is wound on like a ribbon or which is laid in the form of half shells. The external tube is then formed about an insulation by an appropriate means, for example by assembling thin aluminium foils.
The mechanical resistance to crushing of the assembly thus formed is entirely conditioned by the respective strengths of the insulating means and of the external tube, the internal tube only beginning to participate when the insulating means is completely crushed.
In the case of ducts operating at a high temperature (more than 120.degree. C.) and placed in a concrete filled trench which is intended specially for them, the mechanical stresses undergone by the tube are small and such tubes do not need particular mechanical strength in so far as their insulating means and their external tube are concerned.
This latter may even be not sealed without that being serious since, in operation, if water infiltrates accidently into the insulating means, it will be vaporized.
Such a solution, inexpensive in equipment, is on the other hand very expensive from the civil engineering point of view. It has however the advantage of allowing easy maintenance, since it is relatively easy to penetrate into the protective tunnel of the duct for carrying out an inspection therein.
On the other hand, in the case of ducts buried in the ground, they must withstand all the forces transmitted by the ground and in particular those due to the passage of vehicles.
The external tube must necessarily be sealed, since any water penetrating cannot be readily eliminated in the ground and since a wet insulator is a poor insulator.
Two types of solutions exist, each having their drawbacks.
The first consists in using a rigid and solid external tube equipped with centering means for maintaining a constant spacing between the internal tube and the external tube and in placing therebetween a light insulating means. The problem relates to the sealing at the connections of the external tube and the heat bridges which the centering means may form.
The second consists in using an insulating means sufficiently dense to be sufficiently strong mechanically and to cover it with a less resistant sheath. It is the insulating means which absorbs the forces. Since it is dense, it is not very insulating and it must then be of great thickness. It is therefore costly. On the other hand, the problems of sealing and connecting the external sheath are simplified since it may be a simple extruded thermoplastic tube weldable without difficulty at a temperature which does not damage the insulating means.